Biodiesel fuels (BDF) made of a long-chain free fatty acid alkyl ester have begun to spread mainly in Europe, as an environmentally friendly alternative fuel to replace a light oil, because their raw materials are vegetable oil or animal oil, which are natural products. However, in order to be used as an automobile fuel and the like, biodiesel fuels must satisfy high quality standards provided by, for example, ASTM D6751-07b, EN14214:2003, JSI K2390:2008, and EEBS:2008, and must have a high stability.
Typically, a biodiesel fuel is obtained by adding an alcohol such as methanol to a plant-derived oil (fatty acid triglyceride) or the like, inducing transesterification using a homogeneous alkali catalyst such as sodium hydroxide to separate the materials into fatty acid alkyl ester and glycerin, distilling the fatty acid alkyl ester resulting from the separation to remove methanol, mixing the resultant with water to wash the residual homogeneous alkali catalyst, residual glycerin, and residual methanol, separating and removing these impurities dissolved in water together with the water layer, and then drying the resultant to remove the residual water content.
FIG. 1 shows an overview of the above process. However, it is not easy to produce a biodiesel fuel that satisfies fuel standards such as JIS K2390 through such a complicated production process.
Further, a homogeneous alkali catalyst corrodes the production equipment. Therefore, a special equipment is required. What is more, a large amount of waste water is produced through the production process, because sufficient water washing is required for residual metal removal. Furthermore, a lot of waste catalyst will be produced, because the homogeneous alkali catalyst is non-recyclable. Moreover, the homogeneous catalyst will be mixed in glycerin, which is the by-product, which adds to the refinement costs involved in utilization of glycerin.
Further, if a free fatty acid is contained in the raw material oil in a large amount, it reacts with the homogeneous alkali catalyst and produces a soap, which gives rise to a need for a previous step for removing the free fatty acid. Further, in order to prevent saponification and catalyst deactivation, it is necessary that the water content of the raw material be 0.3% or less. As seen above, through such a production method, the biodiesel fuels cannot compete with the light oil fuels in terms of economic efficiency and environmental hazardousness.
To overcome the problems described above, studies are conducted into biodiesel fuel production methods that use, instead of a homogeneous catalyst, a heterogeneous catalyst (solid catalyst) that can be used as it is, i.e., in its solid phase.
For example, studies using an alkali metal-carried catalyst (PTL 1), and an alkaline earth catalyst or a basic solid catalyst (PTL 2, NPL 1) are conducted. However, they have a problem that an active component such as calcium and lanthanum leaches into the produced oil at a high concentration, which tends to reduce the catalyst activity.
Further, according to a method using a zinc aluminate catalyst (PTL 3), high-temperature, high-pressure conditions are required for the reaction, and a residual glyceride level does not satisfy the standards mentioned above.
Furthermore, there is a report (NPL 2) that titania silica (TiO2/SiO2) that has gained wide publicity as an oxidation catalyst for epoxidation, etc., and Ti-MCM-41, which is Ti carried on a mesoporous material MCM-41, are relatively effective for transesterification between low-molecular-weight molecules and an alcohol, which leads to studies into biodiesel fuel production methods with catalysts using a mesoporous material and a microporous material.
However, it has been reported that with a titanosilicate TS-1, the active component tends to leach during the reaction (PTL 4), and that during a transesterification reaction using a titania-containing silica catalyst, the catalyst activity drops in the presence of a free fatty acid and water (PTL 5).
As seen above, various solid catalysts are being developed for production of biodiesel fuels, but all of these have problems. Particularly, with any of these solid catalysts, it is necessary to perform a step of removing a water content and a free fatty acid during transesterification for the sake of an efficient transesterification. Hence, the crucial problem of the homogeneous catalyst cannot have been solved with these solid catalysts.
Under these circumstances, it is requested to develop a solid catalyst with which transesterification of an oil, which is the main reaction, and esterification of a free fatty acid, which is an impurity, can be induced at the same time, and a biodiesel fuel can be produced at a high yield.